1. Field of the Invention
The present invention relates to titanium valves for internal combustion engines and, particularly, to a valve in which the head and stem portions of the valve may be individually fabricated, heat treated and then joined.
2. Background Art
Titanium alloys have long been used to fabricate engine valves for racing and other high performance engines. More recently, titanium aluminides, titanium borides and other titanium intermetallic compounds (TICs) have been used to make engine valves. Titanium is particularly well suited to such applications because of its high strength-to-weight ratio. However, the relatively high cost of titanium has been a deterrent to its use for valves in higher volume passenger car applications where steel remains the valve material of choice. Nevertheless, the performance advantages that can be realized with titanium valves recommend them as an increasingly attractive alternative for passenger car engines.
Aside from the higher material cost of titanium relative to steel, a significant reason for the higher cost of titanium valves is the processing required with traditional fabrication techniques. Titanium alloys used to make engine valves require heat treatments that cause the stem of the valve blank to distort or warp. Although this same problem arises when steel engine valves are heat treated, the distortion of steel valves can be fairly easily removed by mechanical straightening at room or elevated temperature. In the case of titanium alloy valves, the high ratio of titanium's strength to elastic modulus makes this solution impractical. The technique typically utilized with titanium valves is to restrain them from distorting during thermal processing. Such restraint is conventionally provided by suspending the valve on a latticework rack with the stem extending downwardly. Loading such racks is generally done manually and is very time consuming. Moreover, the racks themselves are expensive and tend to warp after several uses due to the elevated temperatures to which they are exposed. Once the racks warp, they no longer provide the intended support, and therefore they must be continually monitored, repaired and/or replaced.
The brittle nature of TICs at room temperature make mechanical straightening impossible. Furthermore, racking as with titanium alloy valves is not a practical solution since the TIC valve blanks will only straighten by vertical racking at very high temperature, and the warpage can occur in the casting or blanking operation. The currently employed solution is to use oversize blanks with the excess material being machined or ground to the desired envelope.
The brittle nature and low strength of TICs at and near room temperature hamper their utility in engines for another reason as well. The various valve actuating mechanisms that are currently employed to open and close valves can apply major bending loads at or near the locating groove emplaced near the top of each valve stem. The primary value of the TICs is to provide strength in the combustion chamber region of the valve (i.e., in the valve head area) where it is hot. Thus, a valve consisting of a strong ductile conventional titanium alloy metal in the upper portion of the stem combined with a TIC in the lower portion of the stem and the valve head would optimize the structural integrity at the valve.
It is known to fabricate steel engine valves in two parts. For example, U.S. Pat. No. 4,073,474 discloses an upset-forged valve in which at least the head and neck portions of the valve are made of a super alloy, and the remaining stem portion of the valve is made of a conventional valve steel. The two portions of the valve are joined by friction welding prior to the forging process. Composite titanium valves have also been proposed. In this regard, U.S. Pat. No. 4,729,546 suggests that the head and stem of a titanium valve may be fabricated separately as distinct preforms which are then inertially welded together. U.S. Pat. No. 4,852,531 discloses a titanium valve having head and stem powder preforms that are joined by cold compaction and then vacuum sintered. In both of the last two mentioned patents, the head and stem portions of the valve are joined within or adjacent to the fillet region. When the resulting valve is installed in an engine, at least a part of the stem portion is exposed below the valve guide and, in the case of an exhaust valve, is exposed to the exhaust gasses.